Neutralizing monoclonal antibodies to the 1957 prototype parainfluenza type 3 virus have been used to map the epitopes on the hemagglutininneuraminidases (HN) surface glycoprotein and to examine the epidemiology of parainfluenza viruses. Competitive-binding assays indicated that these neutralizing antibodies bind to one antigenic site. Analysis of variants selected in the presence of these antibodies showed that there are 5 distinct epitopes within this area. Examination of parainfluenza type 3 viruses isolated from three geographic regions during the past 25 years showed that antigenic variation occurs but this variation is probably a reflection of genetic heterogenicity among these viruses rather than progressive evolution. Furthermore, all of the variation detected in clinical isolates occurs in only 2 of the 5 detectable epitopes on the HN molecule.